4.3.4Unit 4. Rates, Equilibria and Further Organic Chemistry
Enthalpy of Solution – Key Terms — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 4 parts Lesson 26 of 56 Unit quiz
Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Word Equation & Conditions: حمض كربوكسيلي + كحول (في وجود حمض الكبريتيك المركز كعامل حفاز ونازع للماء) ⇌ إستر + ماء
Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
Part 1 of 4The Three Definitions
Each term describes one stage in turning a solid ionic compound into aqueous ions.
Lattice energy
- Lattice energy The enthalpy change when one mole of an ionic solid forms from its gaseous ions under standard conditions, so it is always exothermic.
Enthalpy of hydration
- Enthalpy of hydration The enthalpy change when one mole of a gaseous ion dissolves in water to form one mole of aqueous ions,
- which is always exothermic.
Enthalpy of solution
- Enthalpy of solution The enthalpy change when one mole of an ionic solid dissolves in enough water that further dilution gives no extra heat change.
Standard conditions
- Standard conditions A pressure of 100 kPa, a stated temperature usually 298 K, and each substance in its standard state.
Part 2 of 4Why Energy Is Released or Absorbed
Dissolving can be imagined as two steps even though it happens at once.
1
Energy must be supplied to pull the ions of the lattice apart into separate gaseous ions, which is the reverse of lattice formation.
2
The separated gaseous ions are then surrounded by water molecules, and ion–dipole attractions release energy.
3
Water molecules orient their δ⁻ oxygen toward cations and their δ⁺ hydrogen atoms toward anions.
4
The overall enthalpy of solution is the sum of the energy put in to break the lattice and the energy released on hydration.
5
If hydration releases more energy than breaking the lattice costs, the dissolving is exothermic; otherwise it is endothermic.
Part 3 of 4Comparing the Terms
Comparing the Terms
Sign of lattice energy
- Sign of lattice energy As defined by formation of the lattice it is negative;
- the breaking process has the same size with the opposite sign.
Sign of hydration
- Sign of hydration Forming bonds between water and gaseous ions always releases energy, so hydration enthalpies are negative.
Sign of solution
- Sign of solution It can be positive or negative, depending on which of the two opposing energy terms is larger.
Gaseous ions
- Gaseous ions Lattice energy and hydration both start or finish with separated gaseous ions, which links them in a cycle.
| Term | Starts from | Ends with | Sign |
|---|---|---|---|
| Lattice energy | Gaseous ions | Ionic solid | Negative |
| Hydration | Gaseous ion | Aqueous ion | Negative |
| Solution | Ionic solid | Aqueous ions | Either |
Part 4 of 4Exam Checklist & Tips
4.3.4 – Key Points for the Exam
- Lattice energy (formation) Enthalpy change when one mole of an ionic solid forms from its gaseous ions.
- Enthalpy of hydration Enthalpy change when one mole of gaseous ions becomes aqueous ions.
- Enthalpy of solution Enthalpy change when one mole of solid dissolves in water to give aqueous ions.
- Ion–dipole attraction Attraction between an ion and the partial charge on a water molecule.
Common Mistakes
- Forgetting that the definition of lattice energy specifies gaseous ions as the starting point.
- Giving hydration enthalpy a positive sign; bond formation releases energy.
- Missing the phrase one mole in a definition.
- Believing a compound that dissolves must have a negative enthalpy of solution;
- entropy also drives dissolving.
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